Facioscapulohumeral dystrophy (FSHD) is caused by decreased epigenetic repression of the D4Z4 repeat that results in expression of the DUX4 retrogene. Mutations in SMCHD1 result in decreased D4Z4 epigenetic repression through its direct repressor activity at the D4Z4 repeat and cause FSHD. The broad and long-term goal of this application is to develop therapies for FSHD based on increasing SMCHD1 activity or protein level, or decreasing the activity of factor(s) that counter-act SMCHD1 epigenetic activity at the D4Z4 repeats. The specific goal of the research design is to determine the positive and negative modulators of SMCHD1 activity and their epistatic relationship to the epigenetic repression of the D4Z4 macrosatellite repeat and DUX4 expression. Aim 1 will identify the molecular components and the biological functions of the pathways that regulate the post-transcriptional and post-translational production and activity of SMCHD1, and provide a rational basis for the therapeutic modulation of SMCHD1 in FSHD muscle. Aim 2 will test the hypothesis that FSHD2-associated SMCHD1 variants partially inhibit the activity or stability of the wild-type protein and provide a rationale to target the variant isoforms as a therapeutic approach. Aim 3 Identifies components of the SMCHD1 repressive complex and determines their role in D4Z4 epigenetic repression. Together, these studies will provide the basis for future therapeutic development based on increasing the epigenetic repression of D4Z4 in FSHD individuals.